This invention relates to a gate control circuit for a field controlled thyristor in which the current flowing from an anode to a cathode is controlled by the gate-cathode voltage.
The field controlled thyristor (hereafter referred to simply as FCT) is turned on and brought into the "conducting state" when a negative bias voltage lower than a preset voltage is applied to its gate. In the "conducting state," current flows from the anode to the cathode since the resistance distributed through the anode-cathode path is low. On the contrary, if the negative bias voltage is set higher than the preset voltage, the FCT is turned off to be brought into the "blocking state." In the "blocking state," the current flowing from anode to cathode is substantially blocked.
The above mentioned preset voltage is usually called the "pinch-off voltage" and varies depending on the bias voltage applied between the anode and the cathode of the FCT.
Such a semiconductor device as described above is well-known and disclosed in, for example, U.S. Pat. Nos. 4,037,245 to Armand P. Ferro and 4,060,821 to Douglas E. Houston et al.
The FCTs disclosed in those cited documents have a feature that the current which flows from the anode to the cathode can be controlled by the voltage applied between the gate and the cathode which voltage is set lower than the voltage applied between the anode and the cathode. They also have a feature that when they are turned from the conducting state to the blocking state, a current substantially equal to the current flowing in the conducting state from the anode to the cathode flows through the gate-cathode path, and a feature that a negative bias voltage higher than the pinch-off voltage must continue to be applied to the gate so as to hold the FCT in the blocking state.
In a gate control circuit for an ordinary thyristor or a gate turn-off thyristor, a capacitor is previously charged by a dc power source and then discharged at the time of firing or extinguishing the thyristor. The voltage across the thus discharged capacitor becomes much lower than the voltage across the capacitor developed before the discharge (hereafter referred to for brevity as the initial charge voltage). However, the thyristor need not be supplied with a gate bias voltage once it has been turned on or off. It is therefore not necessary to take the voltage across the capacitor after the discharge thereof into consideration. Accordingly, the capacitance of the capacitor is so selected as to be large only enough for turning on or off the thyristor. In case of applying a gate control circuit using the discharge of a capacitor to an FCT, the voltage across the capacitor must be kept higher than the pinch-off voltage even after the turn-off of the FCT. For, as described above, a negative bias voltage higher than the pinch-off voltage must continue to be applied to the gate so as to hold the FCT in its blocking state.
To maintain the voltage across the capacitor high enough, it is necessary to make the capacitance of the capacitor large or the initial charge voltage high. These measures, however, result in the increase in the size of the capacitor. In general, the physical size of a capacitor is proportional to the electrostatic energy E it can store. The electrostatic energy E is expressed by the following formula: EQU E=1/2CVm.sup.2,
where C is the electrostatic capacitance of the capacitor and Vm the maximum working voltage. It is customary to set the maximum working voltage Vm higher than the initial charge voltage.
A capacitor having a large capacitance and a high working voltage will occupy a rather small space in the gate control circuit assembly if the allowable current of the capacitor is small. However, a capacitor used to turn off an FCT has a large allowable current and occupies a relatively large space in the gate control circuit assembly. It is therefore not preferable to allow the capacitor to have a large capacitance and/or a high working voltage.